Low-temperature starting of gas turbine engines



Nov. 27, 1956 D. P. BARNARD 4TH 2,771,741

LOW-TEMPERATURE STARTING 70F GAS TURBINE ENGINES Filed May 16, 1952 Fuel Line INVEN TOR.

Daniel R Barnar 4M ATTORNEY P/ZIMAIQ y /2 FUEL LOW-TEMPERATURE STARTING OF GAS TURBINE ENGINES Daniel P. Barnard 4th, Chesterton, Ind., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application May 16, 1952, Serial No. 288,355 2 Claims. (Cl. 60-39.06)

This invention relates to improved starting of gas turbine power plants at very low temperatures. More particularly it relates to improved atomization of viscous fuel into the combustion chambers of such power plants at subzero temperatures. 7

The starting of a jet power plant which is equipped with a turbine requires substantially more power than does a piston engine of like output. In general, about ten times the amount of starter horsepower is required to initiate turbo-jet operation. This is largely due to the high compressor speeds needed to pull enough air through the system to make the combustion self-supporting. At very low atmospheric temperatures, starting becomes essentially impossible with the relatively heavy fuels normally used in jet operation because viscous flow through the atomizer nozzles prevents formation of an ignitable fuel spray pattern. The problem of starting aircraft on the ground in Arctic operation has been partially solved by employing a priming fuel such as LPG (liquefied petroleum gas). Such a primlng fuel has been introduced to the combustion zone, simultaneously with but separately from the heavier jet fuel and in sufficient quantity to produce a flame which promotes combustion of the viscous primary fuel when proper atomization of that material has been unobtainable. After combustion of the primary fuel has been initiated by this procedure the flow of LPG into the combustion zone is terminated. Whereas it is relatively convenient to obtain ignition of jet engines in this manner when on the ground, it is not feasible to employ such a technique in the air because the large quantity of LPG required as a priming fuel renders transporting same in an aircraft wholly impracticable. When altitude restarting is required after a flame-out, i. e. combustion failure, caused, for example, by too rich or too lean a fuel mixture or when previously inactivated turbopower plants are needed for added speed c. g. in the target area, dependable high-altitude starting characteristics are essential.

It is a primary object of the present invention to provide improved low temperature starting of gas turbine engines. A further object is to promote satisfactory atomization of viscous jet fuels through the feed nozzles by which such a fuel is injected into the combustion zone of such engines at temperatures as low as 100 F. These and additional objects will be apparent from the following description when considered with the attached drawing.

In accordance with the present invention rapid starting of turbo-jet power plants on the ground as well as high altitude re-starting thereof at low temperatures is assured by introducing, as needed, certain normally gaseous hydrocarbons or mixtures thereof into the primary fuel line leading to the atomizer nozzles in order to reduce the viscosity of the heavier fuel therein and promote satisfactory atomization. Since only that quantity of such material need be introduced to sufficiently dilute the heavy fuel in the line leading to States Patent 2,771,741 Patented Nov. 27, 1956 the nozzles to permit the formation of ignitable fuel spray patterns, a much smaller quantity of the lighter hydrocarbons is required than when such a material is burned as a priming fuel apart from the primary fuel. The use of a relatively small pressure container in the system is therefore entirely adequate for the purpose and operationally feasible.

In the preferred embodiment of the present invention the hydrocarbonaceous material should have sufficient vapor pressure at the low operational temperatures to assure its injection into the primary fuel line without requiring a pump. For this purpose the diluent should have a vapor pressure of from at least about 15 p. s. i. to about 100 p. s. i. at such operational temperature and preferably at least about 25 p. s. i. A pump may, of course, be employed to force the gaseous material into the viscous fuel, in which case, materials having substantially lower vapor pressures at the low temperatures may be used.

The drawing represents a partially diagrammatic sectional side view of a gas turbine jet propulsion engine showing with respect to one of the combustion chambers of such an engine the application of the present invention.

Referring to the drawing and first considering operation of such a power plant under normal temperature conditions, ignition of the engine is initiated by switching on a starter motor (not shown) which is geared to compressor 11. As compressor 11 is rotated, air is drawn through inlet 12 and into combustion chamber 13. For purposes of the present invention the detailed description will be limited to the atomization of fuel into combustion chamber 13 but it should be understood that the invention applies equally to the other chamber shown as well as the others (not shown) which such engines ordinarly embody. Compressor 11 draws in large quantities of oxygen-containing gas and substantially compresses same. With both valve 14 in line 15 leading from storage vessel 19 (for the liquefied normally gaseous hydrocarbonaceous material) and valve 16 in line 17 in normally closed positions, pump 18 is switched on and the jet fuel from tank 20 is pumped through line 21 and nozzles 22 into the compressed oxygencontaining gas in combustion zone 13. The fuel is atomized by nozzles 22, thoroughly intermixed with the compressed gas and ignited by sparkplug 23. The

1 volume of gas in the combustion zone is vastly increased by combustion of the fuel in the presence of oxygen and as the combustion products and unreacted gases leave the combustion zone they impinge on the blades of turbine 24 causing it to rotate at high speed, these gases thereafter issuing from jet 25 at a high velocity.

As turbine 24 rotates it turns shaft 26 which rotates compressor 11, drawing additional oxygen-containing gas into the combustion zone. At this point the starter motor may be turned off and disengaged from compressor 11 since turbine 24 will continue to drive the compressor so long as combustion continues. Spark plug 23 may likewise be switched off after combustion has begun and the temperature is sufiiciently high to promote spontaneous ignition of the fuel-air mixture.

As indicated hereinabove valve 14 is maintained in a normally closed position and when operation of the engine occurs at relatively normal temperatures there is little if any need for opening same to emit gas from pressure vessel 19 into line 21. When, however, very low atmospheric temperatures, e. g. from about 30 F. down to about R, cause viscous flow of the fuel through nozzles 22 resulting in unsatisfactory atomization and/or ignition failure, valve 14 may be opened by the pilot to permit introduction of the gas into line 21, resulting in a reduction in viscosity of the fuel therein and resumption of proper atomization. It is preferred that actuation of valve 14 will cause a metered amount of gas to be introduced to line 21 without requiring the pilots continued attention although this is not neces sary and a manual opening and closing by the pilot is satisfactory. The use of a liquefied normally gaseous hydrocarbon or mixture having a sufficient vapor pressure at the temperature encountered to avoid the necessity of an additional pump is preferred but it should Table 1 Freezing point, not higher than Corrosion (Air-Well Copper Strip) Residue, after evaporation by air jet method, not greater than Aromatics, not to exceed Total sulfur, not to exceed... Gravity, minimum, API Gravity, maximum, API Reid vapor pressure, at 319 0. (100 F.), min... Reid vapor pressure at 37.8" C. (100 F.), max- Water tolerance Accelerated gum content, not even... Distillation range:

10% evaporated, min. ,at 90% evaporated, man, at End point, max Residue, not to exceed Less, not to exceed Bromine number, not to exceed Inhibitor; Following may be added separately or in combination to the fuel in total concentration not over 111). (not including wt. of solvent) per 5,000 U. S. gals. of fuel to prevent gum formation Heat of combustion, net value not less than Mcrcaptan sulfur, not to exceed workmanship No evidence of corrosion and, after B-hr. test period, not more than slight discoloration of the copper strip.

10 mgl100 ml 7 p: s. 1 Substantially immiscible 10 rug/100 ml.

i. with water. Fuel and water layers to be clear and sharply defined; neither to change in color or volume by more than 1 ml.

20 rug/100 m1 204.4" 0. (400 F.) 315.5 o. (600 F.). 1.5

{2,6-Ditertiary Butyl 4- Mo N, N Di secondary Butyl 20 rug/100 ml.

hyl Phenol. Paraphenylenediamine.

2, 4-Dimethyl-6 tertiary-butylphenol.

18,400 BttL/lb 18,400 BttL/lb.

Fuel shall be free from undissolvcd water, sediment, and suspended matter, Odor not to be nauseating or irritating. No substance of known dangerous toxicity under usual conditions of handling and use shall be present.

be understood that a pump may be used in line 15 without departing from the scope of the present invention. It will be recognized in this regard that the usual commercially available LPG which contains: very little if any ethane or ethylene and usually comprises propane and/or butane, would not be suitable at the lowest temperatures, in accordance with the preferred embodimeat of the present invention, since its vapor pressure is insutficient at such temperatures. Such normally gaseous materials may be introduced to the viscous fuel in a slightly different manner, if desired; thus, valve 14 may be closed and valve 16 opened to permit a connection between the gaseous hydrocarbon source and the suction side of pump 18.

The quantity of light hydrocarbon which need be stored in an aircraft for purposes of the present invention is relatively small, viz. that amount needed for one start multiplied by the estimated number of starts expected during an operational flight. Since only that quantity of gas needed to reduce the viscosity of the heavier fuel in the fuel line itself is usually required, it may be seen that a relatively small pressure storage vessel is adequate. An amount of light hydrocarbon equal to from about to about 20% and preferably about to about by weight of the total fuel entering the combustion zone is employed, in accordance with the present invention, during the starting period.

The gaseous hydrocarbon or mixture which may be employed should preferably have sufficient vapor pressure at the particular subzero operating temperature to permit its introduction into the fuel line without a pump. Since a material having vapor pressure greater than about 2000 p. s. i. at 200 F. would require an extremely thick walled vessel and would thus contribute disadvantageously and unnecessarily to the Weight of the aircraft a practical upper vapor pressure limit is inherent.

Mixtures of ethane and propane may likewise be used but relatively slight dilution of ethane can be tolerated before the partial pressure of the mixture becomes too low for use without a pump A mixture comprising 90% ethane and 10% propane has a vapor pressure at l00 F. of about 27.2 p. s. i.; a mixture of 80% ethane and propane has a pressure of about p. s. i., etc. A minimum vapor pressure of about 15 p. s. i. and preferably at least about 25 p. s. i. is necessary in the absence of a pump to assure injection into the jet fuel line. The various hydrocarbon gases having suitable characteristics include ethane, ethylene, propane and propylene, the paraffins being preferred because of their greater stability and availability. Propane and propylene must be used with lighter gases in order to offset their low vapor pressures at low temperatures. The presence of higher molecular weight gases, e. g. butanes or butenes, as impurities is no problem as long as the minimum vapor pressure requirements are adhered to. Similarly, dissolved methane may be present in such amounts that the vapor pressure of the mixture does not exceed limits herein defined.

Set forth in Table l are the specifications of two grades of a hydrocarbon jet fuel derived from petroleum (grades JP-3 and JP-4, as specified under MIL-l 5624A, May 23, 195 1) typical of that with which the above described problems are encountered at the subzero temperatures.

The above description has been primarily directed to providing for satisfactory starts at temperatures as low as about l00 F. but it should be understood that the starting problems described have been encountered at substantially higher temperatures, e. g. from 75 F. to F. Moreover, it should be fully understood that the normally gaseous material employed should be chosen with respect to specific conditions and need not conform to the requirement of having at least about 15 p. s. i. at -100 F. but rather at whatever lowest temperature the engine will be expected to operate.

Although described with particular reference to aircraft gas turbines employed in conjunction with a jet, the present invention is applicable to the solution of low temperature starting problems of the type described, in gas turbines generally, whether air borne or intended for use on the earths surface and whether employed in conjunction with a jet, a propeller, or other means of propuls1on.

Having thus described my invention what I wish to claim as new and useful and to secure by Letters Patent 1s:

1. The method of elfecting starting at low sub-zero temperatures of a gas turbine aircraft engine normally employing a liquid jet fuel which is not readily atomizable and ignitable at low sub-zero temperatures, which method comprises storing in a closed pressure zone separately from the jet fuel a liquefied normally gaseous hydrocarbon having a vapor pressure in the range of about 15 p. s. i. to about 100 p. s. i. at a sub-zero temperature of about 100 F. and introducing normally gaseous hydrocarbon from said closed pressure zone into jet fuel flowing to the gas turbine during the starting period, ef-

fecting the introduction of the normally gaseous hydrocarbon into the flowing jet fuel by pressure developed in the closed zone and controlling the rate of introduction to obtain reduction in jet fuel viscosity and the production of a fuel mixture which is atomizable and ignitable at low sub-zero temperatures.

2. The method of claim 1 wherein the normally gaseous hydrocarbon introduced into the jet fuel flowing to the gas turbine is in an amount in the range of about 5 to 20 weight percent based on total fuel entering the combustion zone during the starting period.

References Cited in the file of this patent UNITED STATES PATENTS 2,520,434 Robson Aug. 29, 1950 2,542,953 Williams, Jr. Feb. 20, 1951 2,579,215 Te Nuyl Dec. 18, 1951 2,612,752 Goddard Oct. 7, 1952 2,628,189 Hawes et al Feb. 10, 1953 2,688,840 Schirmer et al Sept. 14, 1954 OTHER REFERENCES Astronautics, vol. 33, March 1936, pp. 6, 7 and 10. 

1. THE METHOD OF EFFECTING STARTING AT LOW SUB-ZERO TEMPERATURES OF A GAS TURBINE AIRCRAFT ENGINE NORMALY EMPLOYING A LIQUID JET FUEL WHICH IS NOT READILY ATOMIZABLE AND INGNITABLE AT LOW SUB-ZERO TEMPERATURES, WHICH METHOD COMPRISES STORING IN A CLOSED PRESSURE ZONE SEPARATELY FROM THE JET FUEL A LIQUEFIED NORMALLY GASEOUS HYDROCARBON HAVING A VAPOR PRESSURE IN THE RANGE OF ABOUT 15 P. S. I. TO ABOUT 100 P. S. I. AT A SUB-ZERO TEMPERATURE OF ABOUT -100* F. AND INTRODUCING NORMALLY GASEOUS HYDROCARBON FROM SAID CLOSED PRESSURE ZONE INTO JET FUEL FLOWING TO THE GAS TURBINE DURING THE STARTING PERIOD, EFJECTING THE INTRODUCING OF THE NORMALLY GASEOUS HYDROCARBON INTO THE FLOWING JET FUEL BY PRESSURE DEVELOPED IN THE CLOSED ZONE AND CONTROLLING THE RATE OF INTRODUCION TO ABTAIN REDUCTION IN JET FUEL VISCOSITY AND THE PRODUCTION OF A FUEL MIXTURE WHICH IS ATOMIZABLE AND IGNITABLE AT LOW SUB-ZERO TEMPERATURES. 